This invention relates to a gaseous fuel burner assembly, and, in particular, to a line burner assembly for burning a mixture of gaseous fuel and process air. More particularly, the invention relates to a line burner assembly which is able to compensate for variations in the oxygen level in the process air which is mixed with the gaseous fuel to maintain a stable flame during operation of the line burner.
It is known to provide elongated line burners which are formed to include a plurality of gaseous fuel openings and a plurality of air openings along the length of the burner. Such line burners are known as "nozzle mix" line burners. Examples of nozzle mix line burners are shown in U.S. Pat. Nos. 4,340,180 and 4,403,947.
It is also known to supply a premixed gaseous fuel and combustion air mixture to a manifold of a line burner and ignite the mixture to produce a flame. Examples of "premix" line burners are shown in U.S. Pat. Nos. Re. 25,626; 3,178,161; 3,297,259; 4,573,907; and 4,869,665.
Line burners are useful in various industrial applications where it is required to have a specific temperature distribution over a predetermined space or area. Examples of applications where line burners are used include graphics applications, incinerators, turbine boosters, and board dryers. In a graphics application, for example, premix line burners are used to generate hot air to dry ink or solvents from printing presses.
Process air is that air that is produced in a factory or industrial process and found to contain various inert matter entrained therein. It is desirable to dispose of this process air in an environmentally sound way to minimize unwanted discharge of inert matter into the environment. One way to dispose of many of the contaminants entrained in process air is to incinerate it by burning a mixture of gaseous fuel and process air in a line burner. For example, process air containing solvents emitted from a printing press can be introduced into a line burner and mixed with gaseous fuel to produce a flammable mixture. These entrained solvents are incinerated by the flame of the line burner as the process air passes through the mixing region of the line burner and the mixture of gaseous fuel and process air is ignited. It is important that this mixture contain enough oxygen to kindle or sustain a flame.
Problems exist when burning a mixture of process air and gaseous fuel in a burner assembly. Occasionally, the oxygen level in the process air can drop below a minimum acceptable level during operation of the line burner. This drop in the oxygen level in the process air can cause the line burner to become unstable and the flame to be retarded or extinguished. In addition, the oxygen level in the process air is often not capable of supporting the type of high intensity flame which may be required in some applications.
In some instances, the process air stream supplied to a line burner will be low in inerts and relatively high in oxygen and flammable vapors, presenting the burner with a combustible mixture. The line burner can be operated using only a mixture of gaseous fuel and process air in such circumstances. However, in some instances, the process air might not have a composition sufficient to combine with gaseous fuel to produce a satisfactory burnable mixture. This development can lead to disfunction of a line burner set up to burn a mixture of gaseous fuel and process air. The level of inerts and oxygen contained in process air can vary over time so that the quality of the process air does not always contain enough oxygen to support a flame properly when burned.
One object of the present invention is to provide a line burner capable of compensating for intermittent decline in the oxygen level or rise in the inert level in the process air being mixed with a gaseous fuel supply to produce a flame or to maintain the stability of the flame.
According to one aspect of the present invention, a line burner assembly is provided for burning a mixture including at least a gaseous fuel and process air to produce a flame. The assembly includes means for providing a mixing region and means for supplying a gaseous fuel to the mixing region. The assembly also includes means for introducing process air containing oxygen and inerts into the mixing region to mix with the gaseous fuel in the mixing region to produce a mixture. The assembly further includes means for compensating for a decline in the oxygen level in the process air below a predetermined minimum level by introducing combustion air into the mixing region to supplement the process air therein and increase the oxygen level of the mixture above a threshold level to enhance the combustability of the mixture in the mixing region, thereby supporting the flame produced therein.
In the illustrated embodiment of the present invention, the compensating means includes means for supplying combustion air to the mixing region and means for intermittently or periodically activating the supplying means to cause combustion air to be supplied to the mixing region to support the flame produced therein when the oxygen level in the process air falls below a predetermined minimum level. It will be understood that "combustion air" (as used herein) is any air which has a high level of oxygen such that it can mix with gaseous fuel to produce a combustible mixture.
In the illustrated embodiment, duct means is provided for directing process air toward the mixing region of the line burner. Means is also provided for circulating process air through the duct means and into the mixing region so that is mixed with gaseous fuel or a mixture of gaseous fuel and combustion air introduced into the mixing region.
In certain line burner applications, such as the graphics application discussed previously, it can often be predicted when the oxygen level in the process air is likely to fall below the predetermined threshold level required to support the flame. Typically, after firing up the line burner in a graphics application, the oxygen level of the process air will drop below the predetermined threshold level for a known predetermined time period. During this initial fire-up time period, it is advantageous to add combustion air to the mixing region in accordance with the present invention to support the flame. After this initial fire-up time period, however, the oxygen level of the process air typically rises above the predetermined threshold level and is capable of supporting the flame without the addition of any combustion air.
Therefore, in a first embodiment of the present invention, the activating means includes a timer coupled to the combustion air supplying means to control delivery of combustion air to the mixing region so that such delivery occurs at the time when it is needed most. The timer may be set, for example, to activate the supplying means to supply combustion air to the mixing region during the predetermined time period after initial fire-up of the line burner to supplement the process air and gaseous fuel mixture with "oxygen-rich" combustion air during the time when the oxygen level of the process air drops below the predetermined threshold level. After the oxygen level of the process air rises above the threshold level, the timer shuts off the supplying means to stop the supply of combustion air to the mixing region because it is expected that the process air will contain enough oxygen to support a flame when burned with gaseous fuel.
In a second embodiment of the present invention, the activating means includes an oxygen level sensor coupled to the combustion air supplying means to control activation of the supplying means. The oxygen level sensor is located within the duct means to detect the oxygen level in the process air introduced into the mixing region. If the oxygen level in the process air falls below the threshold level, the oxygen sensor activates the supplying means to supply oxygen-rich combustion air to the mixing region. As long as the oxygen level of the process air is above the threshold level, the supplying means is not activated by the oxygen sensor.
In a third embodiment of the present invention, the activating means includes an inert gas sensor. The inert gas sensor is located within the duct means for sensing the level of inert gas in the process air within the duct means. The inert gas sensor is coupled to the combustion air supplying means to control activation of the supplying means. When the level of inert gas in the process air rises above a predetermined level, the inert gas sensing means activates the supplying means to supply oxygen-rich combustion air to the mixing region. As long as the inert gas level is below a predetermined level, the supplying means is not activated by the inert gas sensor so that no combustion air is supplied to the mixing region.
According to another aspect of the present invention, a method is provided for controlling the proportion of process air and combustion air admitted into a line burner assembly. The method includes the steps of providing a mixing region in a line burner assembly and supplying a gaseous fuel to the mixing region. The method also includes the step of introducing process air containing oxygen and inerts into the mixing region to mix with the gaseous fuel in the mixing region to produce a mixture. The method further includes the step of compensating for a decline in the oxygen level in the process air below a predetermined minimum level by introducing oxygen-rich combustion air into the mixing region to supplement the process air therein and increase the oxygen level of the mixture above a threshold level.
Advantageously, an oxygen supplement acts to enhance the combustability of the fuel-air mixture in the mixing region of the line burner, thereby supporting the flame produced therein. Also advantageously, the present invention provides a line burner assembly which can function solely on a mixture of gas and process air when the oxygen level of the process air is above a predetermined level to reduce operation costs for the line burner and to provide for cleaner operation of the line burner.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.